This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2003-47732, filed Feb. 25, 2003, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a semiconductor memory device. More specifically, the invention relates to a semiconductor memory device having a 5-transistor SRAM (static random access memory) cell structure.
2. Description of the Related Art
Recently, in 6-transistor SRAM cells, it has been more difficult to ensure stability as variations in threshold value increase due to microfabrication of transistors. In contrast, 5-transistor SRAM cells are so configured that the sizes and threshold values of transistors making up the cells become asymmetrical and thus have an advantage that its stability is secured more easily than the 6-transistor SRAM cells without increasing the cell size or reducing the cell current. A 6-transistor SRAM cell tends to be configured so as to read data through a single port; therefore, a difference in access speed between the prior art 5-transistor SRAM cell (having a dual port) and the 6-transistor SRAM cell becomes smaller, though the 5-transistor SRAM cell had a great disadvantage in access speed. In the prior art 5-transistor SRAM cells, however, it is difficult to write data xe2x80x9c1xe2x80x9d while keeping data of non-selected cells, and thus the cells are difficult to achieve as an array.
The structure of a prior art 5-transistor SRAM cell will now be described in brief. As shown in FIG. 10, the prior art 5-transistor SRAM cell includes a pair of CMOS (complementary metal oxide semiconductor) inverter circuits 101 and 102 each having a latch structure for data storage and an input/output control transistor (gate transistor) 103 which is connected between the output terminal of the CMOS inverter circuit 101 and a bit line BL and whose gate is connected to a word line WL. The 5-transistor SRAM cell can reduce the number of transistors and that of bit lines by one and thus its area reduction effect is greater than that of the 6-transistor SRAM cell.
The prior art 5-transistor SRAM cell has only one bit line BL. Therefore, the same bit line BL has to be used to write both data xe2x80x9c0xe2x80x9d and xe2x80x9c1.xe2x80x9d
An operation of writing data xe2x80x9c1xe2x80x9d in the prior art 5-transistor SRAM cell will now be described. In xe2x80x9c1xe2x80x9d data write mode, the bit line BL is set at a high (Hi) level to turn on a gate transistor 103 as shown in FIG. 11. If, in this time, the output (Loxe2x86x92Hi) of the CMOS inverter circuit 101 becomes higher than the threshold value of input of the CMOS inverter circuit 102, the output of the CMOS inverter circuit 102 is inverted (Hixe2x86x92Lo). Accordingly, the input of the CMOS inverter circuit 101 is inverted and thus writing of data xe2x80x9c1xe2x80x9d is completed.
The output of the CMOS inverter 101 in xe2x80x9c1xe2x80x9d data write mode depends upon the ratio of on-resistance of the gate transistor 103 to that of a driver transistor (N-type MOS transistor) 101a. It is thus necessary to set the above on-resistance such that the output of the CMOS inverter circuit 101 becomes considerably greater than the threshold value of the input of the CMOS inverter circuit 102. In most cases, however, the on-resistance of the driver transistor 110a has to be set lower in order to ensure the cell current and stabilize the cell. For this reason, conventionally, it was difficult to set the above on-resistance such that the output of the CMOS inverter circuit 101 became very high in xe2x80x9c1xe2x80x9d data write mode.
According to an aspect of the present invention, there is provided a semiconductor memory device comprising first and second CMOS (complementary metal oxide semiconductor) inverter circuits each having a latch structure, a control transistor which is connected between a storage node of the first CMOS inverter circuit and a bit line and whose gate is connected to a word line, and a selection circuit to apply one of a first voltage and a second voltage different from the first voltage to a power supply node of at least the second CMOS inverter circuit, wherein the selection circuit applies the second voltage to the power supply node of the second CMOS inverter circuit at least in xe2x80x9c1xe2x80x9d data write mode.